The ability to capture inorganic matter makes layered double hydroxides (LDHs) and their derivative mixed metal oxides feasible adsorbents of inorganic materials.
From U.S. Pat. No. 5,358,701 a process for removing sulfur oxides (SOx), nitrous oxides (NOx) and chlorine from a gas stream particularly from flue gases of coal-burning power plant by using LDHs is known. The composition of the LDHs was claimed as M1−xIIMxIII(OH)2(An−)x/n.mH2O, wherein MII is an alkaline earth metal, and MIII is selected from a group consisting of IIIA metal cations. Sulfur dioxide (SO2) gas absorbed onto the hydrotalcite structure as SO32− anions by replacing most of the gallery CO32− anions. The spent LDHs were regenerated by calcination at a temperature of 500° C.
U.S. Pat. No. 5,928,496 discloses a process of SO2 adsorption by hydrotalcite-like material. The capacity of the hydrotalcite-like material to adsorb SO2 was enhanced by using acetate anion in interlayer region. Mg1−xAlx(OH)2xA.mH2O hydrotalcite-like material was claimed. Where Mg could be substituted up to 50% by divalent metal cations and Al could be substituted up to 50% by trivalent Cr or Fe metal.
A method for removing elemental sulfur and other sulfur matter from contaminated fluids by hydrotalcite-like material of the formula MgxAly(OH)2x+3y−z(NO3).mH2O is known from U.S. Pat. No. 6,027,636, wherein x, y, z are values from 1 to 6.
In T. J. Toops, M. Crocker, Appl. Catal., B 82, 199-207 (2008) the adsorption of H2S and COS on two calcined LDHs, Mg0.75Al0.25(OH)2(CO3)0.125 and Mg0.65Al0.35(OH)2(CO3)0.175, using Diffused Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and chemisorption apparatus to understand the mechanism of adsorption, is described. Both LDHs adsorbed hydrogen sulfide (H2S) irreversibly, but Mg0.75Al0.25 had a large capacity for carbonyl sulfide (COS).
H. A. J. van Dijk et al., Int. J. Greenhouse Gas Control 5, 505-511 (2011) used calcined synthetic hydrotalcite with a MgO:Al2O3 weight ratio of 70:30 that has been promoted with 20 wt % K2CO3 to enhance the basicity of the mixed oxides. The known adsorbent displayed reversible co-adsorption of CO2 and H2S.
U.S. Pat. No. 7,759,282 discloses a nickel aluminate catalyst for sulfur removal in gaseous stream. Ni2xAl2O2x+3, Ni(2−y)Niy0Al2O(5−y), Ni(4−y)Niy0Al2O(7−y) and Ni(6−y)Niy0Al2O(9−y) catalyst were claimed, wherein X≧0.5 and 0.01≦y≦2.00.
Optimization of mixed metal oxides containing two or three different metals for removing COS was studied by D. E. Sparks et al., Appl. Catal. B, 82, 190-198 (2008). In this known study, different combinations of Ni/Mg/Al, Ni/Mg/Fe, Co/Mg/Al were used for COS adsorption. Ni/Mg/Al mixed oxides had the best performance and a composition with Ni/Mg/Al=0.32/0.48/0.20 showed the best adsorption capacity for COS.
EP 0,986,432 discloses the use of metal ion solution at pH lying in the range of 0.05-7.00 to remove sulfur from gas flow stream. The group of metal ion claimed consists of Zn, Fe, Cu, Ag, Pb, Cd, Co, Mg, Mn, Ni, Sn. The process involved passing sulfur-contaminated gas through a solution containing one of the above stated metals to participate the metal sulfide.
The publication by M. A. Othman et al., J. Hazard. Mater., 254-255, 221-227 (2013) also describes the use of LDHs and mixed metal oxides for removing sulfur from sulfur-contaminated gas or air streams. But here, LDH materials and their calcined mixed metal oxides had been prepared containing either two or three different metals, consisting of the group of Cu, Ni, Zn, Fe, Al, Cr. Ni0.64Fe0.36 mixed metal oxide was found to have the best uptake of H2S.
A. E. Palomares et al., Appl. Catal. B, 20, 257-266 (1999) highlight the potential use of a mixed Co/Mg/Al oxide derived from hydrotalcite-like material for the removal of SOx and NOx in fluid catalytic cracking units. The removal of SO2 was enhanced by cerium oxides where Co-based hydrotalcite-like material was used; whereas Cu-based hydrotalcite did not require the addition of cerium oxides.
In the prior art the majority of studies for selecting the best metals for adsorption of inorganic matter on LDHs have been almost limiting to Mg and Al metals. Therefore, the capability of inorganic matter of the known materials is constricted and following the economical feasibility too. Further, screening studies only let to the use of adsorption materials containing two or three different types of metals within the same structure. The metals were screened hardly taking into account their affinity to the inorganic matter, preferably their sulfur affinity.